Embodiments of the invention relate generally to an MR system and, more particularly, to a balanced mixer for transmitting signals acquired by local RF coils to the MR system for processing.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Generally, the RF coil assembly of an MRI system includes a receive coil used in conjunction with a transmit coil to detect or receive the signals from the excited spins in an imaged object. Typically, each receive coil of the RF coil assembly, also known as a “local coil”, is connected to the receive chain of the MRI system via a coaxial transmission line or cable. Additionally, the receive coils of the RF coil assembly are typically supplied power through the coaxial cables. As the number of receive coils increases, the number of coaxial cables increases to match; thus, a large bundle of coaxial cables results that can become uncomfortable for an imaging patient when laid across the patient and difficult to manage or maneuver.
Further, the connection lines used for transmission are generally directed within a movable patient bed and are therefore several meters in length. Interactions such as parallel resonance and parasitic capacitance between the transmit coil and the coaxial cables can cause standing waves and induced current in the coaxial cables. Current induced in the coaxial cables can cause the coaxial cables to become extremely heated, which furthers patient uncomfortability. To avoid such heat generations, low-attenuation cables may be used for transmission, but these can have large cable diameters, thus resulting in cables that are unwieldy. In addition, large, multi-coaxial connectors that are used to offer means for connecting and disconnecting sections of a multi-cable bundle can be costly and unwieldy in use for the operating personnel.
It would therefore be desirable to have an apparatus and method capable of simplifying the transfer of received MR signals from a local RF coil to the signal processing system of an MRI system.